The present invention relates to a method of providing an improved yield in chlorine dioxide generation processes.
Chlorine dioxide is a selective oxidizing agent widely used in pulp bleaching, water disinfection and numerous other applications. Due to its inherent instability, it cannot be transported and, therefore, it is produced in situ at the point of use.
Commercial methods for chlorine dioxide generation are based on two types of precursor chemicals, namely chloric acid/chlorates and chlorous acid/chlorites.
Large-scale generators typically used in pulp bleaching applications are based on the reduction of acidified chlorate ion solution to chlorine dioxide, according to the following general equation:
ClO3xe2x88x92+2H++exe2x86x92ClO2+H2Oxe2x80x83xe2x80x83(1)
where electrons are provided by means of either chemical reducing agents, such as chloride ion, methanol, hydrogen peroxide, sulfur dioxide, to name only a few, or via an electrochemical route (i.e. cathodic reduction).
For smaller scale applications (e.g. water treatment), the prevailing commercial method of chlorine dioxide generation involves a one-electron oxidation of chlorite ion:
ClO2xe2x88x92xe2x86x92ClO2+exe2x80x83xe2x80x83(2)
whereby the oxidant (electron acceptor) is, again, either chemical in nature (e.g. chlorine, hypochlorite, persulfate, chlorous acid) or electrochemical (anodic oxidation).
Chlorine dioxide generated by means of either chlorate reduction or chlorite oxidation, in most cases, has to be stripped from the reaction medium and reabsorbed in water to form a product solution of a desired strength, prior to being used. In some cases, in particular in certain water treatment applications, the entire process output, including not only chlorine dioxide, but also unreacted precursor chemicals and reaction byproducts, can be directly used in a given application, especially if the chlorine dioxide purity is not of primary concern and/or if the storage of product solution is not feasible for either technical or economical reasons. Generally, however, processes for chlorine dioxide generation typically include a unit operation related to the storage of the product solution.
A general review of industrial processes for synthesis of chlorine dioxide can be found, for example, in W. J. Masschelein""s basic textbook entitled: xe2x80x9cChlorine Dioxide. Chemistry and Environmental Impact of Oxychlorine Compoundsxe2x80x9d, pp. 112 to 145.
In the last several years, a trend has developed in both water treatment and pulp bleaching applications, towards a higher and higher purity of the chlorine dioxide produced. This tendency is primarily driven by the imposition of increasingly stringent environmental regulations related to the minimization of the content of certain types of chlorinated organic compounds, such as AOX (adsorbable chlorinated organics), THM""s (trihalomethanes) and HAA""s (haloacetic acids), believed to be formed due to the presence of chlorine contaminant in chlorine dioxide solution used in pulp bleaching and water treatment.
In view of the above, chlorine dioxide generation processes proceeding according to reaction (1) have evolved from those based on the use of chloride ion as a reducing agent, (known under the trademarks of R2(copyright), R3(copyright), R5(copyright) and SVP(copyright)), which necessarily result in a co-production of substantial quantities of chlorine, to those based primarily on the use of methanol (R8(copyright), SVP-MeOH(copyright), SVP-Lite(copyright)) and hydrogen peroxide (R11(copyright), SVP-HP(copyright)), in which the co-production of chlorine is minimized.
In particular, the subatmospheric, single vessel hydrogen peroxide based process (SVP-HP(copyright)) believed to be described in U.S. Pat. Nos. 5,091,166 and 5,091,167 (both Engstrom at al) and the atmospheric, hydrogen peroxide based process, believed to be described in U.S. Pat. No. 5,380,517 (Sokol), were presented as being superior in terms of the chlorine dioxide purity; see e.g. the recitation in col.3, lines 21 to 24 of the ""167 patent:
xe2x80x9cHowever, with the present process it was possible to produce chlorine dioxide without any substantial addition of chloride ions, thereby obtaining an essentially chlorine-free processxe2x80x9d. (emphasis added)
We found, however, that the strength of the chlorine dioxide product solution originating from the processes of the kind described in the above-mentioned US patents, as well as certain other processes, resulting in the generation of highly pure chlorine dioxide, may significantly decrease over time leading to a substantial decrease in the yield of the overall process. The observed decrease of the product yield can be as low as a fraction of a percent and as high as about 10 to 20 percent. Typically, observed yield losses are in the range of about 0.5 to 5%.
This finding was very surprising since none of the cited prior art documents gave any indication that the high purity of the chlorine dioxide product solution can be detrimental to the overall process yield. The teaching of the prior art was clearly towards achieving the highest possible product purity, with particular emphasis on negligibly low chlorine content.
While the observed phenomenon is not easily explainable in the framework of current knowledge, it is believed that the presence of certain acidic or acidity-generating components in chlorine dioxide (wherein xe2x80x9cacidicxe2x80x9d refers to both Lewis and Bronsted type acids) may have a beneficial effect on the stability of the product solution, thus preventing the decrease of the overall process yield.
It has surprisingly been found that the presence of such acidic or acidity-generating components in the product chlorine dioxide solution to decrease the pH of the chlorine dioxide product solution to less than about 5.5, leads to an improved product stability and hence an overall improved yield.
Accordingly, in one aspect of the present invention, there is provided an aqueous solution of chlorine dioxide originating from a chlorine dioxide generating process, having a purity with respect to chlorine dioxide of at least about 95%, preferably at least about 98%, and acidified by the presence of at least one acidic or acidity-generating component to provide a pH of less than about 5.5. Such chlorine dioxide solution may have a concentration of about 4 to about 15 g/L, preferably about 5 to about 12 g/L, C102.